X-ray tubes used in medical diagnostic imaging are built with a rotating anode structure for the purpose of distributing the heat generated at the focal spot. The anode is rotated by an induction motor consisting of a cylindrical rotor built into a cantilevered axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the x-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is referenced electrically to ground. The X-ray tube cathode provides a focused electron beam which is accelerated across the anode-to-cathode vacuum gap and produces X-rays upon impact with the anode.
In designing the cathode for an x-ray tube, the surface of the cathode is typically desired to be parallel to the target. However, there are instances that the surface between the cathode and the target are not paralleled by design. Unfortunately, if the surface between the cathode and the target is not paralleled, the focal spot will be skewed. The skew of the focal spot could cause the tube to be out of specification, resulting in failure. The focal skew is caused by the electron beams accelerating from the cathode and landing at different coordinates on the target because of the non-parallel surface.
In order to minimize focal skew, it is desirable for the surface between the cathode and the target to be parallel.